Repairing 3D printer heat bed controller

From regular use of the Monoprice Select Mini 3D printer, the heat bed temperature sensor began to fail. The problem was identified as an issue with the wiring attachment to the heat bed thermistor.

During the repair, the heat bed was removed, and unfortunately, in the process, a short across the heating element and the temperature sensor destroyed the electronics on the mainboard. The printer still works, but the heat bed temperature always reads 99 degrees Celsius. The printer starts, tries to print, but heat bead does not heat up.

Option 1 is the most expensive, of course, but would guarantee a fix. Option 2 likely would not work as the issue was caused by actions which voided warranty. Option 3 is a half-way solution, but the mainboard costs about 1/3 the price of the printer, and ships from China which could take months, and includes risk of rejection at import customs.

Option 4 is quite likely the most interesting solution. To make it more interesting, why not build a custom controller? This is what we’ll do.